1. Field of the Invention
The present invention relates generally to a fluid pressure circuit and more particularly to a fluid pressure circuit for controlling automotive automatic transmissions in which a single accumulator may provide a plurality of pressure adjusting characteristics.
2. Description of the Prior Art
In an accumulator for automotive automatic transmissions, various means have been proposed for selectably switching between a plurality of fluid passages to supply various fluid pressures to the accumulator. One such proposed means is disclosed in Nissan Automatic Transmission Service Manual RE5R01A, published in August, 1989. Referring to FIG. 3, the proposed system utilizes a directional control valve 50, and an accumulator 51 which are connected in a fluid circuit. The directional control valve is connected to a high clutch pressure via a port 50a, a reverse clutch pressure via a port 50b, as well as a forward clutch pressure via a port 50c and an accumulator 51 via a port 50d. Further a valve body 50e and a spring 50f, biased in the upward direction of FIG. 3, are also provided.
The valve body 50e is responsive to a pressure P.sub.F from the forward clutch at the port 50c, which acts as a switching pressure to move the valve body 50e within the directional control valve, with or against the spring force of the spring 50f, to communicate either of ports 50a or 50b with the port 50d for supplying either of the high-clutch pressure P.sub.H or the reverse clutch pressure P.sub.R to the accumulator 51.
The accumulator 51 is communicated with port 50d of the directional control valve via a port 51a and further receives a line pressure through a second port 51b. In addition a piston 51c is slidably disposed within the accumulator, the piston 51c is biased by a spring 51d in the upward direction of FIG. 3. Thus, the pressure P.sub.H or P.sub.R is introduced through the port 51a and acts on the piston 51c in cooperation with the spring force of the spring 51d. Opposing this action, line pressure is introduced through accumulator port 51b to act on the piston in the opposite direction. A balance established between these two opposing pressures determines a position of the piston 51c and thereby the pressure adjustment characteristics of either of the pressures P.sub.H or P.sub.R.
In the above described prior art, since there is no relationship between the switching pressure P.sub.F and the pressures P.sub.H or P.sub.R, the pressure adjustment characteristics of both the high-clutch pressure P.sub.H and the reverse clutch pressure P.sub.R are identical, as shown in FIG. 4. However, in the case of the reverse clutch pressure P.sub.R, adjustment characteristics such as shown in FIG. 4 can lead to a time lag arising in certain shift sequences which may further lead to clutch slippage. Therefore, in hydraulic systems such as automatic transmissions, for example, it is preferable that selectively different pressure adjustment characteristics be available. It is further preferable that a plurality of selectively different pressure adjustment characteristics be providable by a single accumulator.